1. Field of the Invention
The present invention relates to a photosensor constituting a photoelectric conversion device used at a light input section of an image information processing apparatus, such as computers, facsimiles, digital copying machines or character readers.
2. Related Background Art
As an element of a photoelectric conversion device used at a light input section of an image information processing apparatus, such as computers, facsimiles, digital copying machines or character readers, a photosensor has been widely used in the art. Recently, a high-sensitivity image reader has been developed which is constructed of an elongated line sensor having photosensors disposed in an array. As examples of a photosensor constituting such an elongated line sensor, there are known a planer type photoconductive photosensor and a so-called sandwich type photoconductive photosensor. Such a photosensor has a photoconductive layer including amorphous silicon (hereinafter abbreviated as A-Si (Hi, X)) containing hydrogen atoms (H) and halogen atoms (X) as a photoconductive substance. In a planer type photoconductive photosensor a pair of electrodes made of such materials as metal are disposed on the photoconductive layer in such a way that a light reception area is formed at a gap between the electrodes. In a so-called sandwich type photoconductive photosensor, the photoconductive layer is sandwiched between a pair of electrodes.
In forming an A-Si (Hi,X) film, various methods have been used, such as a vacuum evaporation method, plasma CVD method, DVD method, reactive sputtering method, ion-plating method, photo-CVD method. Generally, the plasma CVD method among those methods has been widely used in practice.
A photoconductive film made by depositing an A-Si (Hi,X), however has room for further improving the total characteristics including electrical and optical characteristics, fatigue characteristic under repetitive operation, environmental characteristic as well as uniform quality characteristic.
For instance, the reaction process of forming an A-Si (Hi,X) deposition film by a common plasma CVD method is considerably complicated as compared with a conventional CVD method, and closely associated with uncertain reaction mechanisms. Furthermore, in forming a deposition film, many parameters must be taken into consideration, such as substrate temperature, flow and ratio of introduced gases, pressure, high frequency power, electrode structure, reaction room structure, exhaust rate, generation method of plasma, and the like. A proper combination must be selected among the various parameters, and in some cases the plasma becomes unstable and adversely effects the deposition film. In addition, it is necessary to select particular parameters for each apparatus, thus resulting in a difficulty in generalizing the manufacturing conditions.
In spite of such circumstances, the plasma CVD method is presently considered as a most suitable one for forming an A-Si (Hi,X) film which fully meets the requirements of electrical and optical characteristics in various applications.
However, depending on application and usage of a deposition film, it sometimes becomes necessary to conduct mass production with good reproducibility, while fully meeting the requirements of large scale, film thickness uniformity, and film quality uniformity. In such a case, if the conventional plasma CVD method is used for forming an A-Si (H,X) deposition film, the cost of a mass-production apparatus becomes considerably high, and complicated and severe control for mass production as well as fine adjustment of the apparatus becomes necessary. These have been pointed out as the problems to be solved.
Apart from the above problems, a high temperature is required in the prior art using the CVD method, thus making it difficult to obtain a practically available deposition film.
As seen from the foregoing, it has long been desired to develop an A-Si (H,X) film forming method which can conduct mass production inexpensively while retaining practically usable quality and uniformity.
A planer type photoconductive photosensor having a photoconductive layer made of an A-Si(H,X), (hereinafter referred to as A-Si(H,X) photosensor), can produce photocurrent 10 to 100 times as large as that of a photodiode type photosensor. However, there is room for further improvement on (i) light response time and (ii) photocurrent reduction upon application of light.
In addition, since the A-Si(H,X) photosensor utilizes secondary photocurrent (this is the reason why the photosensor can produce large photocurrent as described above), the amount of photocurrent varies with direct relation to the life time of photo-carrier (electron) in the photoconductive layer made of A-Si (H,X),(hereinafter referred to as A-Si(H,X) layer). Therefore, to ensure uniformity of a linear A-Si(H,X) photosensor array made of plural bits disposed in an array, uniformity of electrical characteristics is required for the A-Si(H,X) layer. Such an A-Si(H,X) layer, if manufactured by the conventional plasma CvD method, causes a problem regarding manufacturing yield.